1. Field of the Invention
The present invention relates to a hydrogen storage tank wherein a hydrogen adsorbent, which is capable of adsorbing and retaining hydrogen, is accommodated in a container.
2. Description of the Related Art
In recent years, concern over environmental protection is increasing. From this viewpoint, attention is directed to fuel cell vehicles which use a fuel cell as a running driving source. In addition to the fuel cell, the fuel cell vehicle carries a hydrogen storage tank which stores hydrogen gas to be supplied to the fuel cell.
An example of such a hydrogen storage tank is a high pressure hydrogen storage tank in which hydrogen gas is stored in a container in a state of being compressed at a high charging pressure. In the high pressure hydrogen storage tank, the larger the charging pressure is, the larger the storage amount of the hydrogen gas is. Accordingly, the amount of the hydrogen gas capable of being supplied to the fuel cell is increased due to the large charging pressure. Therefore, in the high pressure hydrogen storage tank, it is necessary to increase the charging pressure to be as high as possible. In order to meet this need, Japanese Laid-Open Patent Publication No. 2002-188794 suggests a high pressure hydrogen storage tank in which the charging pressure increases to about 25 MPa.
However, in the high pressure hydrogen storage tank, it is difficult to improve the charge amount by further compressing the hydrogen gas, because improvement in pressure resistance of the container has a certain limit, and hence the charging pressure also has a certain limit. If the charging pressure is increased, the energy required for the charging is also increased.
In view of the above, a hydrogen storing alloy tank which accommodates, in a container, a hydrogen storing alloy such as LaNi5 alloy, Mg2Ni alloy, and Mg (see, for example, Japanese Laid-Open Patent Publication No. 2002-250593) has been drawing attention. The hydrogen storing alloy is capable of absorbing or occluding the hydrogen gas therein and releasing the occluded hydrogen gas therefrom. Therefore, the charging pressure can be set, for example, to about 1 Mpa, which is a value extremely lower than the pressure used for the high pressure hydrogen storage tank.
The reaction, which is caused when the hydrogen gas is occluded by the hydrogen storing alloy, is an exothermic reaction. Specifically, when the LaNi5 alloy, the Mg2Ni alloy, and Mg are used, large amounts of heat, i.e., 30 kJ/mol H2 (which means 30 kJ per mol of H2, same applies hereinafter), 64 kJ/mol H2, and 73 kJ/mol H2 are generated respectively in order to occlude the hydrogen gas at 25° C. Accordingly, the temperature of the container to accommodate the hydrogen storing alloy is increased, and the occlusion of hydrogen effected by the hydrogen storing alloy is stopped. Therefore, a cooling mechanism is required in order to remove the heat.
On the other hand, the reaction, which is caused when the hydrogen gas is released from the hydrogen storing alloy, is an endothermic reaction. Therefore, for example, in the LaNi5 alloy, it is necessary to supply an amount of heat of 30 kJ/mol H2. In particular, in the fuel cell vehicle, it is necessary to quickly supply the hydrogen gas to the fuel cell in order to obtain the driving force required for sudden acceleration and slope climbing. In order to realize such performance, it is necessary to add a heat exchange mechanism and a heat transfer function as described in Japanese Laid-Open Patent Publication No. 2002-250593.
That is, when the hydrogen storing alloy tank is used, it is necessary to additionally provide, for example, a cooling mechanism and a heating mechanism. Therefore, the weight of the entire fuel cell system is extremely increased.